Refractometer

ABSTRACT

The invention relates to a portable refractometer comprising a depression ( 7 ) for samples, located on an insertion tip ( 11 ) in such a way that once the insertion tip ( 11 ) has penetrated a liquid or a fruit, a sufficient quantity of the sample liquid remains in the depression ( 7 ) for samples, thus wetting a measuring surface ( 4 ) that is delimited in said depression by a transparent body. The refractive index of the wetting liquid can be determined by measuring the intensity of an optical beam that is reflected by the measuring surface ( 4 ).

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a refractometer for determining therefractive index of a liquid and, optionally, variables derivedtherefrom such as, for example, a sugar concentration, having a sensorsystem which has a radiation source for generating a measuring beam, abeam detector for detecting the measuring beam, and a measuring path tobe traveled by the measuring beam, on which path a measuring surface tobe wetted by the liquid and which interacts with the measuring beam issituated.

2. Description of the Related Art

Such refractometers are used as digital or analog measuring instruments,for example, for determining the concentration of certain substancesdissolved in a liquid and influencing the refractive index. For example,in wine-making, the determination of the sugar content in the grapejuice is an important area of application of such measurements.

Two types of measuring instruments are basically known in this area; onerequires a sample to be taken and introduced into the measuringinstrument, for example, drop by drop, while the other design providesfor the immersion of a sensor into the analyte, i.e., into the liquid,to perform the measurement.

Sampling in the case of the first above-named variant is relativelycomplicated and time-consuming, and the instrument must be thoroughlycleaned before each sampling. In the instrument of the second variant,the problem often arises that the refractometer has a temperature whichis different from that of the analyte, so that the required temperaturecompensation in determining the temperature-dependent refractive indexis very difficult to perform. Furthermore, it may be difficult to takeinto account the effect of external light in optical measurements when aprobe is introduced into the substance to be analyzed.

U.S. Pat. No. 5,859,696 describes a refractometer of a type similar tothe one mentioned previously for determining the sugar content in aliquid, in which an optical beam is emitted by a radiation source, isreflected inside a transparent body on a measuring surface, and returnedinto a beam detector. If the measuring surface is wetted on the outsidewith a liquid having a high refractive index, for example, a soft drinkhaving a high sugar concentration, this results in the refractive indexof the liquid on the measuring surface approaching the refractive indexof the transparent body and thus in a weaker reflection of the beam onthe measuring surface or, in other words, in most of the beampenetrating the measuring surface and entering the liquid, so that theintensity of the reflected beam is greatly reduced. This is detectedoptically and a high or low sugar concentration is obtained as afunction of the measured intensity of the reflected beam. The instrumentis usable in a simple manner by immersing a measuring tip into theliquid and detecting the corresponding signal on the portableinstrument. The temperature of the liquid is not taken into account inthe measurement.

Accordingly, it is desirable to refine a refractometer of a simpleconstruction of the above-mentioned type in such a way that a simple andrapid handling is made possible, while effective temperaturecompensation is ensured.

According to the present invention, a refractometer is disclosed inwhich the measuring surface is situated in a depression for samples ofan insertion probe which is insertable into the liquid. The designaccording to the present invention makes it possible to use theinsertion probe, which is insertable into the liquid and containing thedepression for samples, for sampling and measurement, the refractionnumber, i.e., the refractive index, being determined in the depressionafter sampling the liquid to be measured. Only a small volume of liquidremains in the depression for samples; therefore the temperature betweenthe insertion probe and the liquid is rapidly equalized, so that therefractometer and the liquid have the same temperature at the time ofthe measurement. Furthermore, temperature compensation is made simple bymeasuring the temperature at the probe. For this purpose, a temperaturesensor may be placed within the insertion probe.

To take a new sample of the same or a different liquid, it is sufficientto insert the insertion probe into a liquid again; optionally, thedepression for samples may be briefly cleaned beforehand if this seemsto be necessary when handling liquids to be measured. Otherwise, it isalso conceivable to simply introduce the probe into the liquid again,the liquid measured in the first measurement being simply washed out ofthe depression for samples by the second liquid.

Sampling and, optionally also, repeated use of the refractometeraccording to the present invention in a measurement is considerablysimplified, and effective temperature compensation is made possible. Forexample, the insertion probe may have a tip at its end to make insertioninto relatively large fruits possible to directly measure the fruitjuice inside. The surface of the insertion probe may also have a groovethrough which the liquid to be measured may flow into the depression forsamples after brief insertion through the liquid surface or into afruit.

An advantageous embodiment of the present invention provides for themeasuring surface to be delimited by a lens body.

The lens body is situated in such a way that one side is wettable by theliquid and on the other side the lens body surface is kept free of theliquid. The radiation source and the beam detector are then placed onthe side of the lens body which is kept free of the liquid, permittingthe measuring beam from the radiation source to impinge on the lensbody, to be at least partially reflected there on the measuring surfacewetted with the liquid, and to be subsequently directed to the beamdetector. The intensity of the reflected measuring beam is then afunction of the ratio between the refractive index of the lens body andthat of the wetting liquid. The design of the geometry of the lens bodyis preferably such that the measuring beam is bundled or remains bundledin the lens body, and a suitable arrangement of the radiation source andthe beam detector with respect to the lens body may be selected.

According to another advantageous embodiment of the present invention,the measuring surface is delimited by a glass body.

In principle, the fact that the measuring surface is delimited by aglass body means that cleaning of the measuring surface after performingthe measurement is simplified without scratching the measuring surface.In addition, effective temperature equalization between the sensorsystem and the liquid is ensured by the relatively good thermalconductivity of glass.

With respect to the stability of the mechanical construction, thepresent invention is advantageously designed in such a way that theradiation source, the beam detector, and the lens body or, as the casemay be, the glass body, are held in a metallic mount, which is made ofsteel or aluminum in particular.

The design of the mount made of a stable material in the form of steelor aluminum results in the geometrical configuration of the radiationsource, detector, and lens being sufficiently stable to the point thateven impacts will not alter the measuring path. The metallic design alsomakes rapid and effective temperature equalization between theindividual elements of the sensor system and the liquid in thedepression for samples possible. Temperature equalization may beimproved by contact between the liquid and the metallic mount.

It is also advantageous to integrate a temperature sensor into the areaof the sensor system. In this way the temperature of the sensor systemand of the liquid may be measured at the same time as thetemperature-dependent refractive index after these temperatures haveadjusted to one another, which occurs after a few seconds. The measuredrefractive index of the liquid may then be recalculated to a normalizedtemperature in an analyzer, taking into account for the compensation themeasured temperature. Because only a single temperature is to be takeninto account with the sensor system according to the present invention,the calibration of the sensor system is also greatly simplified.

To protect the sensor system in the event of brief temperature changes,the metallic mount may be surrounded by a material, in particular by asynthetic material whose thermal conductivity is less than that of themount material.

Normally the sensor system is protected by a plastic sheath made of apolymer or an elastomer, which of course leaves the depression forsamples and the measuring surface free. The liquid sample taken and thesensor system are largely temperature-equalized independently of theambient temperature due to the thermal contact in the depression forsamples, and the refractive index is measured at this temperature. Thistemperature is measured simultaneously inside the refractometer in thearea of the sensor system, preferably by a temperature sensor, in orderto be able to take temperature influences into account and to refer themeasurement result to a normalized temperature.

According to another advantageous embodiment of the present invention,the lens body or the glass body is made of a material having arefractive index which is greater than 1.5, in particular greater than2. The refractive index of the lens is preferably 1.85.

A high refractive index of the glass body is advantageous when liquidsalso having high dielectric constants or refraction indices are to bemeasured.

The depression for samples advantageously has a volume of less than amilliliter for a particularly rapid temperature equalization.

The sensor system may be advantageously designed in that the radiationsource is formed by an infrared LED and the beam detector is formed by asemiconductor which is sensitive in the infrared range.

Interference by external light is normally very small in the infraredrange, and the components used operate reliably and relativelyunaffected by errors.

The refractometer advantageously has a lens body which has an areahaving greater curvature, which faces the radiation source and the beamdetector, and an area having lesser curvature which delimits themeasuring surface.

This design of the lens body ensures optimal guidance of the measuringbeam and optimum design of the measuring surface in terms of itsmetrological characteristics, the measuring surface also being easy toclean.

The present invention is elucidated in the following on the basis of anexemplary embodiment illustrated in the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically shows the internal structure of the sensor system;

FIG. 2 schematically shows an insertion probe, and

FIG. 3 shows an analyzer of the refractometer according to the presentinvention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

Initially the operating principle of the refractometer according to thepresent invention will be elucidated on the basis of FIG. 1. The sensorsystem having radiation source 1 in the form of an infrared LED, beamdetector 2 in the form of a light-sensitive semiconductor diode and themeasuring path between them is schematically illustrated. A measuringbeam is emitted from radiation source 1 to glass lens body 3 and enters,through its spherical or approximately spherical surface,perpendicularly into the glass body, through which it propagates tomeasuring surface 4, which is formed by a flat delimiting surface oflens body 3. The measuring beam is reflected or partly refracted thereinto the liquid as a function of the ratio between the refractivenumbers (refractive indices) of the material of the lens body and thematerial of liquid 5 which wets the lens body.

At least one portion of the beam may be reflected on measuring surface 4to beam detector 2 and detected by the latter.

The detected radiation intensity is measured using beam detector 2; itis a measure of the refractive index of liquid 5. The measurement iscompared to a reference measurement, which has been made either withouta wetting liquid on lens body 3 or using a known liquid, for determiningthe refractive number.

Due to the small aperture angle of beam detector 2, very little lightreaches beam detector 2 from the outside through wetting liquid 5 andmeasuring surface 4, which permits the influence of external light onthe measurement to be kept particularly low.

Due to the orientation of the lens, the measuring beam suffers littleloss in entering into lens body 3 and exiting to the beam detector,while the flat design of measuring surface 4 makes the entry of externallight into lens body 3 difficult. Cleaning of measuring surface 4 on theoutside, which is exposed to liquids 5 to be measured is alsofacilitated by its flat design. The lens has typically a diameter of 3mm and a refractive index greater than 1.5, in particular greater than2.

Radiation source 1 and beam detector 2 are each situated in bore holeswithin mount 6, which fixedly and reliably determine their position withrespect to one another and to lens body 3.

In addition, mount 6, which is made of metal, for example, steel oraluminum, ensures excellent heat conduction, so that elements 1, 2, 3 ofthe sensor system are reliably kept at the same temperature as mount 6,and the small size of the sample of liquid 5 ensures that the sample isvery rapidly brought to the same temperature as mount 6 via heattransport thanks to glass lens body 3.

FIG. 1 shows, as an example, a small sample in the form of a drop of aliquid 5 on lens body 3; depression for samples 7 may also be regularlyfilled to the rim. In any case, temperature equalization between thesample and mount 6 takes only a few seconds. The depression for sampleshas a volume of less than 1 mL, in particular less than 0.5 mL.

Mount 6 is also provided with a temperature sensor 8 for temperaturemeasurement, which permits temperature compensation when themeasurements are analyzed.

Mount 6 is provided with a plastic layer 19, which insulates itthermally and thus protects the sensor system against varying externalconditions. In the edge area of lens body 3, the lens body is sealedwith respect to plastic layer 19 and mount 6 by elastic seals 20.

The schematic sectional view of FIG. 1 corresponds to a section alongbroken line A—A in FIG. 2, which is described in the following. FIG. 2shows an external view of a portable refractometer having a handle 9,into which a digital display 10 is integrated. An analyzing device whichanalyzes the data delivered by beam detector 2 and temperature sensor 8is mounted in the body of the portable refractometer. The sensor systemis situated in the proximity of insertion tip 11, underneath depressionfor samples 7. In FIG. 2, lens body 3 is shown in the form of a circle.

Insertion tip 11 is designed so that it may be stuck into a fruit sothat the fruit juice contained in the fruit enters depression forsamples 7. However, it is also conceivable that insertion tip 11 isdipped into a liquid and that it has a groove-type notch on the topwhich leads to depression for samples 7 and allows the sample liquid toflow into depression for samples 7 even without insertion tip 11 beingdeeply dipped into the liquid or stuck into the fruit. Otherwise theinsertion tip is introduced into the substance to be analyzed as deep asrequired for the depression for samples to be filled.

The mode of operation of the refractometer is now briefly elucidatedschematically with reference to FIG. 3. Measuring beam 12 is generatedby radiation source 1 in the form of an infrared beam, which propagatesin the schematic representation of FIG. 1 along the central axis ofmount hole 13, represented by a dot-and-dash line, in radiation source 1to measuring surface 4, reflected there, and continues from there alongthe central axis of mount hole 14, also represented by a dot-and-dashline, in beam detector 2. The intensity of the reflected measuring beam12 is measured in beam detector 2. The measured intensity is supplied toanalyzer 15, where a value of the refractive index, i.e., refractivenumber, present is computed by a first computing device 16, initiallywithout taking into account the temperature, using reference values.This value computed from the measurement is then referred to a referencetemperature and thus compensated for the influence of temperature insecond computing device 17, taking into account the temperature valuemeasured by temperature sensor 8 and also provided by analyzer 15. Thevalue of the refractive index, i.e., refractive number, computed andcorrected in this way is then supplied to a display 18 and there outputto the user via a digital display.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

1. A refractometer for determining the refractive index of a liquid andvariables derived therefrom, comprising: a sensor system, said sensorsystem including: a radiation source for generating a measuring beam,and a beam detector for detecting the measuring beam, the measuring beamrunning from the radiation source to a measuring surface to be wetted bythe liquid, at least a portion of the measuring beam being reflected atsaid measuring surface and detected by the beam detector, wherein themeasuring surface is situated in a depression for samples of aninsertion probe which is insertable into the liquid.
 2. Therefractometer as recited in claim 1, wherein the measuring surface isformed by a boundary surface of a lens body.
 3. The refractometer asrecited in claim 1, wherein the measuring surface is formed by aboundary surface of a glass body.
 4. The refractometer as recited inclaim 2, wherein the radiation source the beam detector, and the lensbody are held in a metallic mount.
 5. The refractometer as recited inclaim 4, wherein the metallic mount is surrounded by a syntheticmaterial whose thermal conductivity is less than that of the mountmaterial.
 6. The refractometer as recited in claim 1, wherein atemperature sensor is provided in the area of the sensor system.
 7. Therefractometer as recited in claim 2, wherein the lens body is made of amaterial having a refractive index which is greater than 1.5.
 8. Therefractometer as recited in claim 1, wherein the volume of thedepression for samples is less than 1 milliliter.
 9. The refractometeras recited in claim 1, wherein the radiation source is formed by aninfrared LED and the beam detector is formed by a semiconductor which issensitive in the infrared range.
 10. The refractometer as recited inclaim 1, having a lens body which has an area having greater curvatureand an area having lesser curvature of its surface, wherein the areahaving greater curvature faces the radiation source and the beamdetector, and the area having lesser curvature delimits the measuringsurface.
 11. The refractometer as recited in claim 3, wherein theradiation source, the beam detector, and the glass body are held in ametallic mount.
 12. The refractometer as recited in claim 4, whereinsaid metallic mount is made of steel or aluminum.
 13. The refractometeras recited in claim 11, wherein said metallic mount is made of steel oraluminum.
 14. The refractometer as recited in claim 3, wherein the glassbody is made of a material having a refractive index which is greaterthan 1.5.
 15. The refractometer as recited in claim 14, wherein saidglass body is made of a material having a refractive index which is atleast 2.0.
 16. The refractometer as recited in claim 7, wherein saidlens body is made of a material having a refractive index which is atleast 2.0.